problèmes avec une production en tant que double

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

double prob1(char * s); 
char inbuf[20];

int main()
{
    char inbuf[20];
  double i;

    while (fgets(inbuf, 20, stdin) != NULL)
    {
            i = prob1(inbuf);
            printf("n = %f\n", i);
  }
}

    double prob1(char * s)
    {
        int i,len;
        double digit,num;
        
        i=0;

        len = strlen(s);
        num = 0;
        while (i<len-1)
        {
            
            digit = s[i] - '0';
            num = num * 10 + digit;
            i++;
            
        }
            return num;
    }